[docs][llvm-dwarfdump] Remove unnecessary reference to --show-children

The --show-children option description describes what it does, and
references the =<offset> parameter of section dump switches. I don't
think it needs to be explained again in the documentation of the
section dump switches too.

Reviewed by: JDevlieghere

Differential Revision: https://reviews.llvm.org/D64132

llvm-svn: 365115